Questions & Answers Continued

Why does PlasmaCAM use servo motors instead of stepper motors?

"To me a stepper motor doesn't compare to a servo. If you miss a step you're in trouble. You have to overrate the stepper, and they're a pain to keep accurate."

Experienced machine tool users know that servo motors are superior to stepper motors, because newer, high-performance machines use servo motors, whereas older, more troublesome machines used stepper motors. Yet the real reason for the performance difference requires more explanation.

PlasmaCAM's servo motors use optically-encoded feedback, so the controlling software always knows the true position of the machine. A simple stepper motor controller must "trust" that the motor has moved exactly as requested each time a step current is made. Without feedback, the controller cannot identify and correct whenever the motor misses a few steps (during less than ideal conditions like jolts, vibrations, hang-ups, etc). Since a single shape may require millions of steps to trace, errors in position will continue to accumulate unbeknownst to the controller, until the machine is finally re-zeroed against a physical stop. Hence, errors in stepper position are both unpredictable and unreported.

To solve this problem, optical encoders can be coupled to stepper motors so the controller can determine the true position and make corrections when needed. However, the added cost and complexity prevents the system from being a low-budget alternative to servo systems. The more common solution involves overrating the motors (using larger, higher-geared motors that are under-utilized) to reduce the probability of slipping. This extra rotating inertia (which because of gearing may be much greater than the combined mass of the parts being moved) brings undesirable side-effects: diminished acceleration and speed. As a result, the machine's ability to cut intricate shapes (which require abrupt, harsh changes in direction during high-speed cutting) is greatly limited.

Powerful yet light motors are a must for any plasma table, because their power is needed for speeding up and slowing down all the moving mass, including the spinning motor armatures. (To picture the importance of this, imagine pushing a 100 lb cart at a walking pace and trying to make a sharp 90 degree turn without overshooting at all.) That is why PlasmaCAM fully utilizes the 300 oz-in capability of its servo motors. When combined with the low-mass design of the moving parts in our system, these motors provide over 1.5 G of acceleration. (An object traveling at 100 in/min can totally reverse direction in only 0.002 inch at this acceleration.) To see the obvious benefits of our design, examine the photos of intricate sample parts throughout this web site.

How large is the table?  What size and thickness of material can I cut?
The table and software will cut pieces up to 4ft by 20ft. The actual cutting area of the table is 4ft by 4ft - but after cutting the first 4ft area the table pauses, the sheet is indexed, and cutting is resumed until the entire shape is cut.

"We cut a 7½ ft sign for our band and it was simple. We didn't even follow the directions."

Maximum cutting thickness depends on your plasma torch, and the machine can accept material up to ¾" thick or more. However, thick plate is very heavy so we recommend only working with smaller pieces of it.

Why doesn't PlasmaCAM sell larger machines?
We don't offer larger machines for several reasons. If you're looking for a larger machine, consider the following tradeoffs and ask yourself if size is really worth compromising what matters most:

• The vast majority of plasma cut parts are less than 4 ft in size and can be cut on the PlasmaCAM table without indexing. Though material is usually bought in 8 or 10 ft lengths, sheets are much easier to move through a smaller table than a larger table.
• Material handling is much more difficult with larger machines. For this reason, high-dollar machines utilize complex drive mechanisms and pallet changers to allow easy crane or forklift access to material. Without these luxuries, getting large sheets in and out of a big machine is harder than indexing the same sheet through the PlasmaCAM table, with rollers at each end.
• Larger machines are not as accurate. As size increases, accuracy decreases because tolerances are harder to hold on larger parts. Alignment of parts in a large assembly is also more difficult and complex, requiring extensive field work. The PlasmaCAM machine achieves the highest accuracy because it's the optimum size.
• Larger machines have lower performance. They require heavier moving parts that cannot move as fast. The result is low productivity and poor quality when cutting lighter materials. By contrast, PlasmaCAM uses lightweight yet rigid parts that move quickly.
• Larger machines are much more expensive and complex. Not only are many larger, additional parts required, many indirect costs become higher too. The cost of shipping, installation, operation and maintenance all increase drastically with size. We find that most customers prefer an affordable, trouble-free machine over a large machine.
• Economies of scale make the PlasmaCAM machine very affordable, because we build so many of them. Research has shown that If we were to sell a larger machine too, we would only be able to sell about 1/20th as many. The cost of supporting this extra product would be more than the benefit - both to the company and to our customers.
• Everyone has a different idea of how big a machine should be. If we tried to make every possible size, we would not be able to focus on making a really good machine like we do now.

After examining the all physics and economics of size, we have concluded that a larger machine could not be made to work as well as the PlasmaCAM system and still be affordable.